1. Field of the Invention
The present invention relates to a beam scanning apparatus. More particularly, the present invention relates to a synchronism detecting mechanism for detecting a scanning position of a beam.
2. Description of the Prior Art
In a beam scanning apparatus incorporated in a laser beam printer, a facsimile, etc., a larger beam radiated by a semiconductor laser device is deflected by a rotary polygonal mirror (polygon mirror) at an isometric speed. The laser beam reflected is directed to a scanning line (a photoreceptor) to form an image after the scanning speed thereof is corrected by an f.theta. lens system. On each scanning operation, the laser beam is irradiated on a beam receiving device (hereinafter referred to as "SOS sensor") arranged at a position equivalent to the scanning line in order to detect a beam position. The reason why the SOS sensor is provided at a position equivalent to the scanning line is that a high synchronism detection accuracy is obtained at the equivalent position since scanning is performed under a condition where aberrations are removed and beam is converged.
In recent years, since a demand for a compact beam scanning apparatus has been increasing so that a degree of freedom for arranging the SOS sensor has been decreasing, or since one beam scanning apparatus is used for a plurality of printers, the SOS sensor is inevitably arranged at a position inequivalent to the scanning line. As a result, measures are required for securing a necessary synchronism detection accuracy for the SOS sensor provided at an inequivalent position.
The following are the factors which determine synchronism detection accuracy:
(1) beam diameter on the SOS sensor; PA1 (2) response speed and sensitivity of the SOS sensor and beam quantity; and PA1 (3) scanning speed of beam on the SOS sensor.
An output waveform of a sensor when beam passes on the SOS sensor is a substantially square wave, and the inclination of a rising portion of the output waveform affects the detection accuracy (the inclination is preferably steep).
As a prior art where the SOS sensor is arranged at a position inequivalent to the scanning line, Japanese laid-open Patent Application No. S54-140548 is known. In the synchronism detection apparatus according to the prior art, in order to improve detection accuracy, the SOS sensor is arranged beyond a position equivalent to the scanning line, and beam is re-converged by a lens in front of the sensor to increase scanning speed. However, when the lens arranged in front of the SOS sensor is a spherical lens, it is difficult to correct aberrations. Because of this, there arises a problem that beam is not excellently converged on the SOS sensor when the beam misses the center of the lens due to errors in installing the lens or other elements, or a due to thermal deformation of the housing.